The production of synthesis gas from the solid and liquid carbonaceous fuels, especially coal, coke, and liquid hydrocarbon feeds, has been utilized for a considerable period of time and has recently undergone significant improvements due to the increased energy demand and the need for clean utilization of otherwise low value carbonaceous material. Synthesis gas may be produced by heating carbonaceous fuels with reactive gases, such as air or oxygen, often in the presence of steam in a gasification reactor to obtain the synthesis gas which is withdrawn from the gasification reactor.
The synthesis gas can also be used to generate power from otherwise environmentally unacceptable fuel sources, and as a source of feed gas for the synthesis of hydrocarbons, oxygen-containing organic compounds or ammonia.
Synthesis gas mixtures comprise carbon monoxide, carbon dioxide, and hydrogen. Hydrogen is a commercially important reactant for hydrogenation reactions.
Other trace materials often found in the synthesis gas include hydrogen sulfide, ammonia, cyanides, and particulates in the form of carbon and trace metals. The extent of the contaminants in the feed is determined by the type of feed and the particular gasification process utilized as well as the operating conditions. As the product gas is discharged from the gasifier, it is usually subjected to a cooling and cleaning operation involving a scrubbing technique wherein the gas is introduced into a scrubber and contacted with a water spray which cools the gas and removes particulates and ionic constituents from the synthesis gas. The initially cooled gas may then be treated to desulfurize the gas prior to utilization of the synthesis gas.
When the desired product is hydrogen, the synthesis gas from the gasifier is advantageously further processed by water-shifting, also called steam reforming, using catalyst to form hydrogen from carbon monoxide as shown below:
H2O+COxe2x86x92H2+CO2
The water shift process, or steam reforming, converts water and carbon monoxide to hydrogen and carbon dioxide. The shift process is described in, for example, U.S. Pat. No. 5,472,986, the disclosure of which is incorporated herein by reference.
The hydrogen gas is often used in subsequent refining processes, particularly hydrotreating. For many applications, especially for hydrotreating hydrocarbons, the hydrogen is required at higher purity than is available in synthesis gas or even water shifted synthesis gas, and at pressures between about 1000 psi and about 3000 psi. The shifted or unshifted synthesis gas must therefore be purified to meet product specifications. In addition, the purified gas may need to be further compressed.
Relatively pure hydrogen at high pressure can be obtained from synthesis gas via the pressure swing absorption process. This method is expensive and requires significant capital outlay.
What is needed is an efficient and cost effective method of extracting a relatively pure high pressure hydrogen stream from synthesis gas.
The present invention is a process to recover a high purity, high pressure hydrogen gas stream from synthesis gas, and to efficiently recover and utilize the low grade carbon monoxide and dioxide gas that is the byproduct of the hydrogen purification. The synthesis gas is contacted with a membrane that separates the synthesis gas into a hydrogen-enriched permeate and a hydrogen-depleted non-permeate. The permeate is conveyed to a carbon dioxide absorber. The carbon dioxide absorber removes carbon dioxide using a solvent. The carbon dioxide-rich solvent from the absorber is heated and sent to a gas-liquid contactor, where the solvent is regenerated by nitrogen stripping. A small recycle stream of a regenerating gas, i.e., hydrogen, is subsequently contacted with the solvent, stripping entrained and dissolved nitrogen from the solvent. This stripping gas, the regenerating gas, or preferably both, are then mixed with the non-permeate for combustion in a combustion turbine.